The present invention relates to a drive unit for driving a movable object incorporating mainly an optical component, such as a lens being used in a camera, a DVD drive, a CD drive, an MD drive, an endoscope or the like.
As a drive unit that uses an electromechanical transducer, such as a piezoelectric element, which changes its dimension (expands or contracts) when a voltage is applied, the drive unit shown in an exploded perspective view in FIG. 1A and in an assembled perspective view in FIG. 1B, for example, is known (U.S. Pat. No. 6,016,231).
A piezoelectric element 4 comprises multiple piezoelectric plates stacked. One end 4a thereof in the expansion/contraction direction is secured to a base 1, and the other end 4b thereof is secured to the first end 5a of a rod (supporting member) 5. The rod 5 is slidably supported on supporting sections 2 and 3 integrated with the base 1.
The rod 5 is held between the main unit 10a and the cap 10b of a movable object 10, and a hold-down spring 10c exerts an urging force to the main unit 10a and the cap 10b in the holding direction. The movable object 10 is thus supported so as to be slidable along the rod 5.
A voltage control circuit is connected to the piezoelectric element 4. For example, when a predetermined drive voltage having a rectangular waveform or the like is applied to the piezoelectric element 4, the piezoelectric element 4 vibrates with a displacement, the shape of which is nearly the same as that of the drive voltage. Because the rod 5 is flexible, the rod 5 vibrates with a displacement having a sawtooth shape in the direction of its length in response to the displacement of the piezoelectric element 4 (see FIG. 2).
More specifically, in period A, the piezoelectric element 4 expands relatively slowly in the gradually rising inclined section 101 of a first vibration displacement 100, and the rod 5 moves slowly in the direction indicated by arrow I shown in FIG. 1B. Next, in period B, the piezoelectric element 4 contracts abruptly and its length returns to its initial length (as shown by the waveform section indicated by the falling inclined section 102). The rod 5 thus moves abruptly in the direction indicated by arrow II.
Movements similar to those described above are repeated hereafter. As a result, the rod 5 repeats the slow movement in direction I and the abrupt movement in direction II, thereby vibrates. Hence, the rod 5 vibrates with a displacement having a sawtooth vibration waveform characterized by a gradual rise and a rapid fall as shown in FIG. 2.
The spring force of the hold-down spring 10c of the movable object 10 (the frictional connection force applied from the movable object 10 to the rod 5) is adjusted so that the movable object 10 moves together with the rod 5 when the rod 5 moves slowly (in period A), and so that the movable object 10 stays where it is due to inertia (or moves by an amount smaller than that of the rod 5) when the rod 5 moves abruptly (in period B), as shown in FIG. 3. Hence, the movable object 10 moves in direction I relatively with respect to the base 1 while the rod 5 vibrates.
In the case that the movable object 10 is moved in the direction indicated by arrow II shown in FIG. 1B, the vibration waveform of the rod 5 should be the reverse of that shown in FIG. 2, that is, a waveform having a rapidly rising section and a gradually falling section. The movement principle of the movable object 10 in this case is similar to that of the above-mentioned case. In this case, the vibration displacement of the rod can be reversed from that shown in FIG. 2 by changing the duty ratio of the rectangular wave applied to the piezoelectric element 4.
The movable object 10 of the drive unit can be moved relatively with respect to the base (stationary base) 1. Hence, the drive unit can be used as the lens drive unit of a camera, for example. In other words, with a configuration in which the movable object 10 is connected to a lens frame, the lens can be moved simultaneously with the movement of the movable object 10.
In the case that the drive unit that uses such a piezoelectric element having the movement principle described above is used to move an optical member, the positional control of the movable object 10 becomes important. In the positional control of the movable object 10, the position of the movable object 10 should be detected at all times so that the movable object 10 can be stopped accurately at a position immediately before the end of the movable range. However, it is difficult to accurately carry out positional control for the drive unit that uses such a piezoelectric element. Hence, the supporting section 3 that supports the rod is used as a restricting member that restricts the movement range of the movable object, and the movable object is securely made contact with the supporting section 3 that restricts the movement of the movable object (see FIG. 4A).
However, in the case that the movable object is made contact with the restricting member (the supporting section 3) and stopped, if the driving of the piezoelectric element is not stopped immediately after the movable object is made contact with the restricting member, there is a problem in which the movable object is tilted as shown in FIG. 4B. In other words, in the case of a drive unit that is actually used in the lens barrel of a camera or the like, the diameter of the rod is appropriately 1 mm, but the diameter of the lens frame 11 connected to the movable object is appropriately 10 mm. Hence, the sizes and weights of the members extending from the movable object are larger than those of the movable object. For this reason, if the state in which the movement of the movable object 10 is restricted using the restricting member 3 continues while the piezoelectric element 4 is driven to move the movable object 10, the movable object 10 is apt to be tilted. Although the tilting of the movable object is very slight, in the case of the drive unit connected to an optical member, such as a lens 12, the tilting becomes a cause of significant deterioration in the performance of the optical system.
For the purpose of solving this problem, the restricting member is made larger or a sensor capable of accurately detecting the position of the movable object is provided, whereby it is possible to prevent the tilting of the movable object. However, such a configuration in which the restricting member is made larger and a sensor is provided is not desirable from the viewpoint of downsizing the optical system, such as a lens barrel, in recent years. Furthermore, providing a sensor causes a problem of increasing cost.
Accordingly, for the purpose of solving the technical problem described above, the present invention is intended to provide a drive unit capable of preventing the tilting of the movable object thereof in the case that the movable object is stopped at the position where the movable object makes contact with the restricting member.